An ingredient in antibacterial soaps can help arson investigators solve crimes, according to research presented Sunday at the American Chemical Society national meeting in San Diego. Chemists have found that if they coat soil samples from a crime scene with the antimicrobial agent triclosan, they can preserve gasoline residue, which is a chemical signature for arson.

When crime-scene investigators suspect arson, they collect soil at the scene of the fire. But sometimes those soil samples sit for weeks or months before they are analyzed, says Dee Ann Turner, a graduate student at Indiana University-Purdue University Indianapolis (IUPUI). During that time, microbes break down gasoline and alter its chemical fingerprint. In some cases, investigators can’t determine if arson caused the fire because the evidence has degraded.

To study microbial degradation of gasoline in soil, Turner and IUPUI analytical chemistry professor John V. Goodpaster first needed to simulate arson. To do so, they enlisted help from a firefighter, who threw gasoline-filled beer bottles on the ground. After the fire burned out, the researchers collected soil in paint cans, covered half the samples with a 2% solution of triclosan in 0.2 M sodium hydroxide, and sealed the cans. After 60 days, the researchers analyzed the volatile components of the samples using gas chromatography/mass spectrometry.

In the untreated soil, the researchers found an altered and unpredictable ratio of five chemicals that are typically used to fingerprint gasoline. But the triclosan-treated soil retained the normal gasoline signature. The signature was still evident when the researchers let the soil sit for 140 days.

Freezing soil samples might also stop the degradation, Turner says, but that’s not always practical. The best way to stop degradation, she adds, is to preserve the evidence when it is collected.